While there are a large number of heat exchangers or chemical reactors capable of effecting indirect heat exchange between two fluids, e.g. in the simple heating and cooling of a first fluid by a second fluid or in the thermal induction of a reaction in a first fluid by heat supplied by a second fluid or for the abstraction of reaction heat of the first fluid by a second fluid, problems continue to be encountered, especially when the reactor must be operated at high temperatures and with elevated pressures.
In such cases, significant technological problems arise. for example, when the operating temperature must be 900.degree. to 1300.degree. K., or in a similar range, the components of the heat exchanger must be composed, at least at the so-called hot end, of special alloys which are expensive and frequently have limited strength. When in addition, the fluids are under pressure, the problem is accentuated since the wall thickness of any stressed parts must be increased, thereby raising the cost of the unit.
The heat exchanger must have large dimensions to be practical for a particular chemical or other process, the reactor or heat exchanger cannot be designed from standard tables because of the higher wall thickness, special materials, etc. In fact, some of the refractory alloys which may be required are not conveniently available in large quantities. The weight of the reactor can become prohibitive in that the unit can no longer be readily transported by standard commercial means.
This applies to heat exchangers having a cylindrical housing and a tube bundle therein to which one of the fluids is admitted at one end and removed at the other, to heat exchangers in which a fluid is admitted to the housing around a tube bundle at one end and systems in which access to the space within the housing is effected through the cylindrical wall of the casing or shell.
In a coiled-tube exchanger, moreover, while it is common practice to wind the coils of the tube bundle on a core tube or pipe by rotating the later between end supports, the size of the heat exchanger which can be fabricated is limited by the weight of the tubes which can be supported by the tubular core. In practice, the coil continues to a point just before buckling of the core tube and this buckling point limits the size of the heat exchanger bundle which can be formed on the tube.
Naturally, it is possible to increase the mass of pipe which can be carried by using a core tube of larger diameter, but this requires a trade-off since it increases the useless volume, i.e. the space occupied by the core tube.